In digital image processing systems, each pixel is typically represented by a numeric value specifying the color of that element. These contone (color) pixel levels are generally continuous from one pixel to another. In some systems however, there are additional numeric values called “tags” associated with each pixel, which may describe how the pixel is to be rendered. For example, these tags (known also as “rendering intents” or “rendering hints”) may specify to the rendering stage which halftone is to be used for a printing application. In contrast to the contone (color) pixel levels, where color values are continuous, adjacent tag values are in general not related. For example one pixel on the border of one object may have a tag value of 5 (150 cluster dot) associated with it. An adjacent pixel, which is associated with another type of object may have a tag value of 6 (300 line screen). The tag values of 5 and 6 are close, but the rendering hint they each refer to provide significantly different rendering outputs.
If the digital image processing system produces the image in its original format, i.e., including all pixels, each pixel is rendered according to the correct tag. However, in some applications, it may be necessary to filter the original image prior to rendering. For example, a user may wish to print a “draft” version or lower resolution version or a user may wish to print a monochrome version rather than a full color version, etc. General filtering (when an image is being reduced in resolution or size, for example) involving the contone data is very straightforward, as the final pixel color is simply a weighted sum of the surrounding pixel color values. However, filtering of the tag data plane presents a far more difficult problem, due to the lack of correlation between tag levels. Applying a weighted sum of the surrounding pixel tags may not produce a clear answer if there are tag values of 5 and 6 present in the filtering window. If there are tag values of 1 and 9 in the filter window, an entirely inappropriate tag may be chosen.